This invention relates to die attach adhesive compositions. More specifically, it relates to die attach adhesive compositions containing at least one bisbenzocyclobutene monomer.
A die attach adhesive bonds the inactive side of a semiconductor chip to a semiconductor package. During assembly of the semiconductor package, the die attach adhesive holds the chip firmly in place during wire bonding and encapsulation. After assembly, the adhesive provides a conductive path to remote heat from the chip and to provide an electrical ground. Examples of commonly used die attach adhesives are eutectic solders, conductive epoxies, and conductive polyimides.
Eutectic solders are metal alloys typically made with gold. A "preform", which is a metal foil cut to the shape and size of the semiconductor chip, is deposited on the desired substrate of the package and is heated to a temperature near the melting point of the preform. The chip can then be placed onto the preform with a scrubbing motion. Although eutectic solders are very reliable and exhibit outstanding thermal stability, they are expensive and difficult to process.
Conductive epoxies are typically low viscosity pastes containing at least 70 percent of an electrically conductive metal, typically silver. The epoxy is applied to the substrate of the package by conventional means and the chip is then placed in contact with the coated substrate. The epoxy can then be cured in one step. Epoxies are less expensive than eutectic solders, are easy to process, and exhibit excellent adhesive strength. However, they possess poor thermal stability at high temperatures and an unacceptable coefficient of thermal expansion at high temperatures, properties which are necessary for advanced applications in electronics.
Conductive polyimides are similar to conductive epoxies. They possess acceptable adhesive strength, excellent thermal stability at high temperatures, and an acceptable coefficient of thermal expansion. Unfortunately, polyimides must be dissolved in a high boiling solvent, such as N-methylpyrrolidone, for use as a die attach adhesive. The solvent must be evaporated during a two-step cure. This results in a substantial weight loss during cure and can lead to void formation under the semiconductor chip. Voids can reduce adhesive strength and thermal conductivity, problems which become more serious as the chip size increases.
Copending U.S. application Ser. No. 005,189 (filed Jan. 20, 1987) discloses die attach adhesive compositions comprising an arylcyclobutene monomer (more commonly referred to as a "cyclobutarene" monomer) and an electrically conductive metal. The compositions are easy to process, exhibit outstanding thermal stability at high temperatures, and exhibit an acceptable coefficient of thermal expansion at high temperatures. Furthermore, unlike conductive polyimides, solvents are unnecessary and weight loss on cure is minimal. However, the adhesive strength of these conductive arylcylobutanes is less than the adhesive strength of the polyimides.
In view of the deficiencies of the prior art a die attach adhesive composition, that is easy to process, exhibits excellent adhesive strength, has thermal stability at high temperatures, and an acceptable coefficient of thermal expansion, is needed. Furthermore, a composition, that does not require any solvent, initiator, or curing agent for use as a die attach, does not require a two-step cure, and does not exhibit a substantial weight loss on cure, is needed.